In fixed roof type storage tanks for storing petroleum products or the like therein, a gas-phase space is formed above a liquid (liquid phase) stored in the storage tank. For example, when a temperature inside the storage tank rises, the amount of evaporation of a gas from a surface of the liquid is increased to increase a pressure of the gas in the gas-phase space. In order to prevent an excessive stress from being applied to a main body of the storage tank along with the pressure rise, a relief valve for venting the gas present in the space to the outside when the pressure of the gas becomes higher than a predetermined pressure is provided to a roof of the storage tank (for example, FIG. 1 of Patent Literature 1).
Some petroleum products and the like, which are liquids in the storage tank, contain volatile organic compounds (VOC) such as benzene and toluene corresponding to volatile compounds. In this case, a VOC recovery unit for recovering the VOC contained in the gas vented through the relief valve is sometimes provided in terms of environmental conservation or the like.
On the other hand, the temperature inside the storage tank varies from day to day in accordance with weather conditions. Even during a single day, the temperature inside the storage tank and the pressure in the gas-phase space momentarily change in accordance with the presence/absence of sunlight during day or night or a change in the amount of sunlight along with movement of the sun during the day. Therefore, it is difficult to precisely obtain the gas emission amount vented through the relief valve. Therefore, hitherto, capacity of treating the emission gas in the VOC recovery unit has been designed based on an instantaneous emission amount through the relief valve, which is defined in industry standards such as American Petroleum Institute (API) 2000.
However, specifications of the relief valves defined in the industry standards are determined based on the amount of evaporation of the gas, which is generated under the most severe conditions, without consideration of a difference in area where the storage tank is installed and thermal conduction properties (for example, with or without heat retention) of the main body of the storage tank. Therefore, in comparison to an actual gas emission amount, a capacity of the VOC recovery unit installed together with the storage tank tends to be excessively large.